Field of the Invention
The invention relates to a layered semi-finished product, for further processing into fibre-reinforced composite components, and to a method for producing the semi-finished product.
Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Fibre-reinforced composite materials, consisting of technical textiles and thermoplastic plastic materials, are known from prior art. Also known are flat semi-finishes, such as prepregs and semipregs, which consist of textile and thermoplastics layers interconnected to one another, but not yet fully mutually consolidated, or organic sheets. Producing such semi-finishes usually takes place in continuous processes, for instance in double belt presses, utilizing pressure and temperature, involving at least partially fusing the thermoplastic and connecting it to the textile. This pre-consolidation results in restrictions regarding the use of varying thermoplastics, layer thicknesses, or fibre layers and fibre orientations, since adhesion among the individual layers might be influenced by this pre-consolidation.
Semi-finishes of this kind usually serve the users for producing complex structures, for example in the areas of vehicle construction or aerospace technology. In order to produce one finished component made of fibre-reinforced composite material, usually several of these semi-finishes are stacked one on another in or on a mould. Due to the existing firm bond between thermoplastic and textile layers on account of a glue or a partial consolidation, at this stage the cold formability of the semi-finish is greatly restricted. This requires heating up the semi-finish to the thermoplastics' melting temperature already during preforming, that is, even before the actual forming step, in which the semi-finishes are consolidated and hardened in themselves and with each other by means of increasing their temperature.
The terms “to consolidate” and “consolidation” mean in this context that the fibre-reinforced composite material is hardened (cured). This may happen in part (partial consolidation) or completely. The fibre-reinforced composite material is said to be unconsolidated when no hardening process has occurred.
These two heating processes, which result from the structure of current semipregs and prepregs are not only energetically unfavourable, but also result in trapping more air between the individual layers as the semi-finishes are joined together. This may have a negative effect on both mechanical properties and aesthetics of the finished component.
To avoid such air pockets, the semi-finishes are often joined together at reduced pressure.
In this context, publication WO2001/000405 A2 describes a method wherein a partly consolidated prepreg is evacuated in and heated a vacuum bag to remove air trapped between the prepreg layers. The heating, taking place simultaneously with air removal, ensures a complete saturation of the fibrous material with the thermoplastic, and finally a consolidation of the layers resulting in the finished component. Also in this case, the pre-consolidation of the prepreg results in poor cold formability, which makes premelting during preforming necessary. In addition, the entire technical equipment and other expenses for evacuating air is still to be provided by the end user.
Enclosing the semi-finishes, and possibly also the mould, in a vacuum bag, in any case means increased labour and increased technical effort for the user. In order to make air evacuation of the layer structure as simple as possible for the end user, some semi-finishes are already known from prior art that have an optimised structure for evacuating air in a vacuum bag.
In this context publication EP 2 085 212 A1 describes a multi-layered, fibre-reinforced flat thermoplastic material, the core of which contains layers having venting openings or ducts, so that the trapped air may escape better during compression and consolidation of the flat material. However, the individual layers of the multi-layered flat material are in this case firmly joined to each other; for instance, the cover layers are pressed on after heating them, thus glueing them to the remaining flat material. The ensuing pre-consolidation results in a poor cold formability of the flat material. Furthermore, the evacuation of air from the layers, together with, or shortly before the forming step, still involves complex technical equipment.
To disburden the end user, some methods are already known from prior art in which producing the semi-finishes takes place under vacuum, thus evacuating them already during production.
Publication DE 41 15 831 A1 describes a continuous method for producing flat composite materials, whereby a layered flat material is processed into a tubular film. The individual layers made from thermoplastics and a fabric of reinforcing fibre are contained in a string of continuous tubing the edges of which are firmly joined together, in parallel to the feed direction. Evacuation of the continuous tubing, respectively of the layered flat material then takes place when the stacked layers are passing through a press. In this press the tubular film is sealed at a right angle to the feed direction, and the matrix thermoplastic is heated and melted. Evacuation of the flat material thus takes place at the same time as its consolidation or pre-consolidation. In the first case, the complex evacuation of the prepreg is a matter for the user; in the second case, the result is yet again a pre-consolidated semi-finished of poor cold formability.
In addition, known semi-finishes are not suitable, or only in a very limited manner, for producing components with complex geometries. Sometimes the problem arises that the semi-finishes, when placed in a press, or upon closing the press in order to consolidate them, shift, or do not fit evenly due to their complex geometry.
None of the semi-finishes known so far from prior art combines all those properties necessary for producing complex components with an excellent spectrum of characteristics. These characteristics include in particular: good drapability, avoiding fibre slip, and preventing air pockets in the semi-finished product. This applies equally to the prepegs known from prior art, layered structures made from fabrics of reinforcing fibres which have been pre-impregnated using matrix materials, and the already known semipregs, layered structures having superficially joined layers made from fabrics of reinforcing fibres and matrix material.